Thermal Comfort and the Heat Stress Indices

Industrial Health 2006, 44, 388–398

*To whom correspondence should be addressed.

Review Article

Thermal Comfort and the Heat Stress Indices

Yoram EPSTEIN* and Daniel S. MORAN

Heller Institute of Medical Research, Sheba Medical Center, Tel Hashomer and the Sackler Faculty of Medicine,

Tel Aviv University, Israel

Received January 16, 2006 and accepted April 13, 2006

Abstract: Thermal stress is an important factor in many industrial situations, athletic events and

military scenarios. It can seriously affect the productivity and the health of the individual and

diminish tolerance to other environmental hazards. However, the assessment of the thermal stress

and the translation of the stress in terms of physiological and psychological strain is complex. For

over a century attempts have been made to construct an index, which will describe heat stress

satisfactorily. The many indices that have been suggested can be categorized into one of three groups:

“rational indices”, “empirical indices”, or “direct indices”. The first 2 groups are sophisticated

indices, which integrate environmental and physiological variables; they are difficult to calculate

and are not feasible for daily use. The latter group comprises of simple indices, which are based on

the measurement of basic environmental variables. In this group 2 indices are in use for over four

decades: the “wet-bulb globe temperature” (WBGT) index and the “discomfort index” (DI). The

following review summarizes the current knowledge on thermal indices and their correlates to thermal

sensation and comfort. With the present knowledge it is suggested to adopt the DI as a universal

heat stress index.

Key words: Heat stress, Thermal sensation, Comfort, Thermal indices, Heat balance

Heat Balance and Heat Exchange

An essential requirement for continued normal bodyfunction is that the deep body temperature will be maintainedwithin a very narrow limit of ± 1°C around the acceptableresting body core temperature of 37°C. To achieve this,body temperature equilibrium requires a constant exchangeof heat between the body and the environment. The rateand amount of the heat exchanged is governed by thefundamental laws of thermodynamics. In general terms,the amount of heat that must be exchanged is a function of:a. the total metabolic heat produced, which for a 70 kgyoung male, may range from about 80 watts at rest to about500 watts for moderately hard industrial work (and up to1,400 watts for a very trained endurance athlete); b. theheat gained from the environment (≈17.5 watt per changeof 1°C in ambient temperature, above or below 36°C). Theamount of heat that can be exchanged is a function of sweatevaporation (≈18.6 watt per 1 mmHg change in ambientvapor pressure, below 42 mmHg (assuming a mean skin

Introduction

Workers, soldiers, and travelers are often exposed tosevere environmental heat stress, which may deterioratework efficiency and productivity and may even threatensurvival1–6). It is thus expected that the physiological heatstrain experienced by an individual will be related to thetotal heat stress to which he is exposed, serving the need tomaintain body-core temperature within a relatively narrowrange of temperatures. Many attempts have been made toestimate the stress inflicted by a wide range of workconditions and climate, or to estimate the correspondingphysiological strain and to combine them into a singleindex—a heat stress index. The difficulties in creating auniversal heat stress index are outlined in the present reviewand a simple way to indicate the level of the environmentalheat stress is proposed.

389HEAT STRESS INDICES

temperature of 36°C)).The basic heat balance equation is:

∆S= (M–Wex) ± (R+C) –E ........................ (eq 1)

Where: ∆S = change in body heat content; (M–Wex) =netmetabolic heat production from total metabolic heatproduction (Wex=mechanical work); (R+C) =convective andradiative heat exchange; E=evaporative heat loss.

In the situation of thermal balance ∆S=0, then:

(M–Wex) ± (R+C) =Ereq ............................. (eq 2)

This form defines the required evaporation to achievethermal balance (Ereq).

Noteworthy, evaporative capacity of the environment isin most of the cases lower than Ereq; and thus, the maximalevaporative capacity of the environment (Emax) should beconsidered. The ratio Ereq/Emax, which denotes the requiredskin wettedness to eliminate heat from the body, is a “heatstrain index” (HSI) that was proposed by Belding and Hatch7).The singular equations of Ereq and Emax are beyond thescope of the present discussion; but, to solve these equationsseveral parameters should be measured and eventually theinteraction between them will define the human thermalenvironment.

The Six Agents of Heat Stress

It follows from the heat balance equation that ambienttemperature per se is seldom the cause of heat stress; it isonly one, and rarely the most important, of several factorsthat compose the term “heat stress”8). According to Fanger,the interactions of six fundamental factors define the humanthermal environment and its sensation of thermal comfort9).These parameters are subcategorized into environmentalfactors and behavioral factors (Table 1). Ambienttemperature, radiant temperature, humidity, and air movementare the four basic environmental variables; the metabolicrate and clothing (insulation and moisture permeabilitycharacteristics) provide the behavioral variables that affecthuman response to thermal environment. Thus, anyconsideration of thermal stress should explore these sixfactors.

Tradeoffs have been established between these six factorswith respect to their effects on human comfort and infer theeffect of five on ambient temperature (Ta)8, 10):

Metabolic rate: an increase of 17.5 watt (above restinglevel) is equivalent to a 1°C increase in Ta.

Clothing insulation (clo): a change of 1 clo is equivalentto a change in 5°C at rest and 10°C while exercising.

Radiant temperature (MRT): a change of 1°C in MRTcan be offset by a 1°C in Ta.

Wind speed: a change in 0.1 m/sec in wind speed isequivalent to a change in 0.5°C in Ta (up to 1.5°C).

Humidity: a 10% change in relative humidity can be offsetby a 0.3°C in Ta.

How these tradeoffs affect the comfort temperature canbe illustrated by the following example: for an individualwho is exposed in the sun at 22°C (assuming this is thecomfort temperature, see next section) with an increase inMRT of 5°C and who is dressed in clothing with 2 clo theequivalent air temperature (the weighted temperature towhich an individual is exposed to) is 32°C. Under theseconditions the comfort temperature will be 12°C (for furtherdetails cf Ref 10).

Thermal Comfort

Thermal comfort is defined as: “that condition of mindwhich expresses sa t is fact ion wi th the thermalenvironment”11, 12). According to this definition comfort isa subjective sensation. Based on ASHRAE definition thezone of thermal comfort is the span of conditions where80% of sedentary or slightly active persons find theenvironment thermally acceptable13). In terms of climaticconditions the acceptable ambient temperature of comfort

Table 1. The 6 key factors in determining thermal comfort

parameter symbol also

Environmental

1. Dry-bulb temperature (Ta) To

To =0.5(Ta+MRT)

(To ≈2/3Ta+1/3Tg )

2. Black-globe temperature (Tg) MRT

MRT=(1+0.22V0.5)(Tg–Ta)+Ta

3. Wind velocity (V)

4. Wet-bulb temperature (Tw) rh; VP

behavioral5. Metabolic rate (M) met

6. Clothing

Insulation (clo)

Moisture permeability (im)

The conversions in this table are based on Shapiro and Epstein8).

rh=relative humidity, VP=vapor pressure, met=a metabolic rate unit (1

met=50 kcal/h/m2), To= operative temperature, an index of the combined

effects of dry bulb and radiant temperature; first degree estimate for a

sunny clear day is: To=Ta+5 (°C).

(For further information on terminology and units, the interested reader

is referred to the Glossary of terms for thermal physiology44))

390 Y EPSTEIN et al.


would be slightly higher in the summer than in the winter,being 23–27°C and 20–25°C, respectively13).

Fanger (1970) defined 3 parameters for a person to be inthermal comfort: a. the body is in heat balance; b. sweatrate is within comfort limits; c. mean skin temperature iswithin comfort limits9). These conceptual requisites fordetermining thermal comfort can be expressed by measurableterms as: body-core temperature within a very narrow rangeof 36.5–37.5°C, a skin temperature of 30°C at the extremitiesand 34–35°C at body stem and head, and the body will befree of sweating1, 14). Any deviation from these assertionsresults in sensation of discomfort. In reference to equation1, thermal comfort will be attained when the rate of heatdissipation from the body by means of radiation andconvection (cardiovascular tone) will equal the rate ofmetabolic heat production and, consequently, heat storage(∆S) will be nil. In other words heat stress results fromimbalance between the demands imposed on the worker bythe task and the environment, and the worker’s capacity toeliminate the heat load as modified by clothing15). It followsthat thermal comfort is directly related to sweat evaporation.This can be expressed by the ratio of demand to capacity(Ereq/Emax). As this ratio exceeds 0.2 (20%), the worker ismoved from a “comfort” condition to “discomfort”. As theratio increases to 0.4–0.6, the worker is subject toperformance decrements. Above 0.6, work will be usuallydiscontinued or will be performed for only a limited periodand above 0.8 there is substantial risk of heat illness15).

Thermal sensation and thermal comfort are bipolarphenomena ranging from “too cold” to “too hot” with comfortor neutral sensation in the middle. This continuum ofsensations has been described by several scales9, 11, 12, 16, 17).The subjective ratings of discomfort and the correspondingphysiological correlates are summarized in Table 2.

Throughout the twentieth century and into the twenty firstcentury there has been an active research on: what conditionswill produce thermal comfort and how to grade heat stress.These efforts resulted in various models attempting todescribe thermal comfort. These studies have not beenconducted only for their scientific merit but rather to establishsafety limits and to increase productivity.

Indices for Assessing Heat Stress

A heat stress index is a single value that integrates theeffects of the basic parameters in any human thermalenvironment such that its value will vary with the thermalstrain experienced by the individual18). In 1905 Haldanewas probably the first in suggesting that the wet-bulb

temperature is, as a single value, the most appropriate measureto express heat stress19). Since then a large number of indiceshave been suggested; about 40 indices are listed in Table 3,and there are probably many others, which have beensuggested and are (or were) in use throughout the world.At first the purpose of the index was limited to the estimationof the combined effect of environmental variables. Later,the effects of metabolic rate and clothing were also takeninto account. Noteworthy, the efforts to assess heat stressby a single value that will combine several variables continue,although already in the 1970s’ Belding and then Gagge andNishi concluded that there cannot be a universal valid systemfor rating heat stress, mainly because of the number andcomplexity of interaction of determining factors20, 21).

To be applicable an index must meet the followingcriteria22):

a. feasible and accurate at wide range of environmentaland metabolic conditions.

b. consider all important factors (environmental,metabolic, clothing etc).

c. relevant measurements should reflect the worker’sexposure, without interfering with his performance.

d. exposure limits should be reflected by physiologicand/or psychological responses reflecting increasedrisk to safety or health.

Heat stress indices can be divided into 3 groups, accordingto their rationale18, 22): indices that are based on calculationsinvolving the heat balance equation (“rational indices”),indices that are based on objective and subjective strain(“empirical indices”), and indices based on directmeasurements of environmental variables (“direct indices”).Obviously, indices of the first two groups are more difficultto implement in work places, since they evolve too manyvariables and some of them require invasive measurements.The third group of indices is more friendly and applicablesince these indices are based on monitoring environmentalvariables.

The most comprehensive indices are those that are basedon the heat balance equation (“rational indices”). Theseindices integrate all environmental and behavioral variables,which have been stated above. However, since there is nopractical way to record all the elements that are required tosolve the heat balance equation some of the parameters areassumed or regarded as constants. Such is the case, forexample, with the “heat stress index” (HSI) that was proposedby Belding and Hatch and is based on a constant skintemperature of 35°C7).

It appears that over the years too much emphasis has beenplaced on the academic accuracy of an index at the expense


of practicability. In reality, the prevailing conditions in workplaces are not uniform, as they are under laboratoryconditions. In such a case work is performed under varyingdegrees of physical work load, heat stress, and work periods.Other confining factors may be different types of clothing,gender, degree of acclimatization age, etc. Therefore, inthe writers’ view, the use of a “direct index” together withappropriate, simple, and practical guidelines accounting forwork intensity and clothing is the preferred way of expressingthermal stress.

The “Direct Indices”

The ability to construct safety regulations become rathercomplex, if simultaneously four environmental parameters,a work rate, and a specified clothing level has to beconsidered. It is, therefore, imperative to consider simplifiedways of obtaining an estimate of thermal balance, causingchanges in body heat content. This can be achieved by usingan index that is based on direct measurements ofenvironmental variables, which is used to “simulate” heatstrain.

Following this concept Houghton and Yaglou proposedalready in 1923 the effective temperature (ET)23). This indexwas originally established to provide a method fordetermining the relative effects of air temperature andhumidity on comfort. In 1932 Vernon and Warner substitutedthe dry-bulb temperature with a black-globe temperature toallow radiation to be taken into account (the “corrected

effective temperature” (CET))24). Since then manymodifications were made to this basic index. For the presentdiscussion two indices, which are in daily use for many yearsare regarded.

The wet-bulb globe temperature (WBGT) index:The wet-bulb globe temperature (WBGT) is by far the

most widely used heat stress index throughout the world. Itwas developed in the US Navy as part of a study on heatrelated injuries during military training25). The WBGT index,which emerged from the “corrected effective temperature”(CET)24) consists of weighting of dry-bulb temperature (Ta)wet-bulb temperature (Tw) and black-globe temperature (Tg),in the following manner:

WBGT=0.7Tw+0.1Ta+0.2Tg ................... (Eq. 3)

For indoor conditions the index was modified as follows:

WBGT=0.7Tw+0.3Tg .............................. (Eq. 4)

( f o r i n d o o r p u r p o s e s , w h e n T g≈T a, t h e nWBGT=0.7Tw+0.3Ta)

The coefficients in this index have been determinedempirically and the index has no physiological correlates;but, it was found that heat casualties and the time lost dueto cessation of training in the heat were both reduced byusing this index. This index is recommended by manyinternational organizations for setting criteria for exposingworkers to hot environment and was adopted as an ISOstandard (ISO 7243)26–31).

Table 2. Comfort vote and thermal sensation, in association to the physiological zone of

thermal effect and the associated percent skin wettedness

Vote Thermal sensation Comfort sensation Zone of thermal effect HSI

(a) (b) (c) (d) (e) (f)

9 Very hot Very uncomfortable Incompensable heat 80

+3 8 hot uncomfortable 40–60

+2 7 warm Slightly uncomfortable Sweat evaporation 20

+1 6 Slightly warm compensable

0 5 neutral comfortable Vasomotor compensable 0

–1 4 Slightly coolShivering compensable

–2 3 Cool Slightly uncomfortable

–3 2 cold

1 Very cold uncomfortable Incompensable cold

Based on: Goldman45) and Shapiro and Epstein8).

a. Thermal scale according to ASRAE 5511).

b. Thermal scale according to Rohles17).

f. The “heat strain index” (HSI) is the ratio of demand for sweat evaporation to capacity of

evaporation (Ereq/Emax)7). This denotes also the percent of skin wettedness, which is a good predictor

of warm discomfort46).



Table 3. Proposed systems for rating heat stress and strain (heat stress indices)

Year Index Author(s)

1905 Wet-bulb temperature (Tw) Haldane19)

1916 Katathermometer Hill et al.47)

1923 Effective temperature (ET) Houghton & Yaglou23)

1929 Equivalent temperature (Teq) Dufton48)

1932 Corrected effective temperature (CET) Vernon & Warner24)

1937 Operative temperature (OpT) Winslow et al.49)

1945 Thermal acceptance ratio (TAR) Ionides et al.50)

1945 Index of physiological effect (Ep) Robinson et al.51)

1946 Corrected effective temperature (CET) Bedford52)

1947 Predicted 4-h sweat rate (P4SR) McArdel et al.53)

1948 Resultant temperature (RT) Missenard et al.54)

1950 Craig index (I) Craig55)

1955 Heat stress index (HIS) Belding & Hatch7)

1957 Wet-bulg globe temperature (WBGT) Yaglou & Minard25)

1957 Oxford index (WD) Lind & Hellon34)

1957 Discomfort index (DI) Thom36)

1958 Thermal strain index (TSI) Lee & Henschel56)

1959 Discomfort index (DI) Tennenbaum et al.39)

1960 Cumulative discomfort index (CumDI) Tennenbaum et al.39)

1960 Index of physiological strain (Is) Hall & Polte57)

1962 Index of thermal stress (ITS) Givoni58)

1966 Heat strain index (corrected) (HSI) McKarns & Brief59)

1966 Prediction of heart rate (HR) Fuller & Brouha60)

1967 Effective radiant field (ERF) Gagge et al.61)

1970 Predicted mean vote (PMV) Fanger9)

Threshold limit value (TLV)

1970 Prescriptive zone Lind62)

1971 New effective temperature (ET*) Gagge et al.63)

1971 Wet globe temperature (WGT) Botsford64)

1971 Humid operative temperature Nishi & Gagge65)

1972 Predicted body core temperature Givoni & Goldman66)

1972 Skin wettedness Kerslake67)

1973 Standard effective temperature (SET) Gagge et al.68)

1973 Predicted heart rate Givoni & Goldman69)

1978 Skin wettedness Gonzales et al.70)

1979 Fighter index of thermal stress (FITS) Nunneley & Stribley71)

1981 Effective heat strain index (EHSI) Kamon & Ryan72)

1982 Predicted sweat loss (msw) Shapiro et al.73)

1985 Required sweating (SWreq) ISO 793374)

1986 Predicted mean vote (modified) (PMV*) Gagge et al.75)

1996 Cumulative heat strain index (CHSI) Frank et al.76)

1998 Physiological strain index (PSI) Moran et al.77)

1999 Modified discomfort index (MDI) Moran et al.78)

2001 Environmental stress index (ESI) Moran et al.79)

2005 Wet-bulb dry temperature (WBDT) Wallace et al.80)

2005 Relative humidity dry temperature (RHDT) Wallace et al.80)


Based on the WBGT index the American Conference ofGovernment Industrial Hygienists (ACGIH) published the“permissible heat exposure threshold limits values” (TLV),which refer to those heat stress conditions under which nearlyall workers may be repeatedly exposed without adverse healtheffects28). These criteria were adopted also by theOccupational Safety and Health Administration (OSHA) andthe American Industrial Hygiene Association (AIHA)29, 30).The American College of Sports Medicine (ACSM) and theUS Army published guidelines of exercising under variouslevels of heat stress31, 32) (see appendix).

Yet, inherent limitation of the WBGT is its applicabilityacross a board range of potential scenarios and environments,because of the inconvenience of measuring Tg. The black-globe temperature is measured by a temperature sensor placedin the center of a thin copper matt-black globe (diameter:150 mm). In many circumstances measuring Tg iscumbersome and impractical22, 33).

The discomfort index (DI)The question arises to what extent the black-globe

temperature is essential in the determination of environmentalheat stress. In 1957 Lind and Hellon proposed the “Oxfordindex” (WD)—a simple direct index based on a weightedsummation of aspirated wet-bulb temperature (Tw) and dry-bulb temperature (Ta) in the following form34):

WD=0.85Tw+0.15Ta ............................... (Eq. 3)

The weighting of 85% of the effect on Tw appears to reflectman’s reliance on sweat evaporation for temperatureregulation in hot environment. The reflection of this indexon physiological strain was demonstrated by showing a veryhigh correlation with the physiological tolerance time (timeto reach rectal temperature of 39.2°C and/or heart rate of180 bmp), for resting unclothed men35). Though this indexis easy to use it was argued that it is not appropriate wherethere is significant thermal radiation18). Nevertheless, becauseof its easiness to use under field/industrial conditions, thisapproach is appealing and other indices, which are basedon the same concept were proposed (Table 4).These indicesdiffer from each other by the relative weight of the twocomponents to the index value, compensating in part forthe lack of measuring the effect of radiant temperature.

Recognizing that the WBGT is still the index adopted byinternational authorities, we correlated the six indices in Table4 with the WBGT index, using a randomly set ofmeasurements (n=108). It is obvious that all indices highlycorrelate to the WBGT with r2 values that range from 0.930to 0.967.

From the six indices that are presented in Table 4, theindex that is of special interest is the discomfort index (DI),which is the only index, beside the WBGT that is in dailyuse for more than 4 decades. The DI was originally proposedby Thom36) and was slightly modified by Sohar et al, asfollows37):

DI=0.5Tw+0.5Ta ...................................... (Eq. 4)

In its present form (eq. 4), The DI was found to be highlycorrelated to the effective temperature (ET) index38). Moreimportantly, the DI correlated to sweat rate both at rest andunder exercise, reflecting its physiological significance39).The DI values are very similar to those of the WBGT indexas depicted in Fig 1, with an r2 value of 0.9466 and even ahigher correlation with no y intercept (r2=0.999; SE=0.0035).

Based on a great number of observations on a widespectrum of population groups and under different climaticconditions, the following criteria were established tocharacterize the environmental heat stress and the correlatethermal sensation: under DI values of 22 units no heat stressis encountered. Between 22–24 units most people feel amild sensation of heat; between 24–28 units the heat load ismoderately heavy, people feel very hot, and physical workmay be performed with some difficulties. Above 28 unitsthe heat load is considered severe, and people engaged inphysical work are at increased risk for heat illness (heatexhaustion and heat stroke)40, 41). The Israel Defense Forces(IDF) and the Israeli Ministry of Education adopted thisclassification and published accordingly guidelines forexercising in the heat. For example, guidelines for fluidconsumption can be set by work intensity and the grade ofheat stress42).

Depending on the application the use of the DI enablesthe determination of the heat load at any given time. It mayalso be expressed as a daily minimum and maximum as wellas the total daily heat load or sub-divided to mild moderate

Table 4. “Direct indices” that are based on wet-bulb and dry-

bulb temperatures

Index Formula

Oxford index (WD)34) 0.85Tw+0.15Ta

Discomfort index (DI)36) 0.4Tw+0.4Ta+8.3

Discomfort index (DI)39) 0.5Tw+0.5Ta

Fighter index of thermal stress (FITS)71) 0.83Tw+0.35Ta+5.08

Modified discomfort index (MDI)77) 0.75Tw+0.3Ta

Wet-bulb dry temperature (WBDT)79) 0.4Tw+0.6Ta

The values calculated by these indices highly correlate with the

values of the WBGT index, with r2 values of 0.930–0.967.



and severe heat load. Equally, the data can be calculatedfor a month, season, or even a whole year37, 43). To illustratethe use of the DI the mean hourly heat load in two cities inIsrael were compared: Tel Aviv, which has a Mediterraneanclimate (hot/humid) and Beer Sheba, which is situated onthe edge of the desert. The profile of the heat load in Augustis different in Tel Aviv than in Beer Sheba40, 43). In August,for example, there are in Tel Aviv 12 h of moderate heatload, 8 h of mild heat load and only 4 h without any heatload. In Beer Sheba there is a mean of nine hours daily ofmoderate and three hours of mild heat load; during 12 hthere is no heat load at all. This shows that the climate inBeer Sheba is more comfortable than in Tel Aviv althoughthe mean monthly ambient temperatures in Beer Sheba ishigher than in Tel Aviv (31.2°C vs 29.5°C, respectively)40).By connecting all locations that show similar pattern of heatload a climatologic base-map of a country or an area can bedrawn36, 40). From a biometeorological perspective this formis more logic than to describe ambient temperature andhumidity separately.

Presenting heat stress in terms of environmental heat loadwith the appropriate physiological significance and theappropriate TLVs or other safety measures is a step forwardin increasing productivity and reducing health hazards relatedto heat stress. Likewise, describing an area by the prevailingheat load is beneficial in comparing regions based on thermal

comfort (i.e. using of air conditioning etc). For bothapplications the DI was found to be very satisfactory in Israel.Its application worldwide deserves further investigation.

Summary

During the last century agronomists, physiologists, andbiometeorologists have attempted to propose an index thatwill accurately define heat stress and the zones of discomfort.These efforts were not purely academic but rather to establishsafety criteria for workers who are exposed to heat stress(metabolic or environmental). The many indices that wereproposed could be grouped into “rationale indices”,“empirical indices”, and “direct indices”. While the first 2groups are sophisticated indices, which require for theircalculation the use of many physiological and environmentalfactors, the third group is based on the measurement of basicenvironmental variables. It is apparent that the “directindices” and the “empirical indices” are more comprehensivethan the “direct indices”, but their practicality in daily useis questionable. Therefore, it is in the writers’ mind that asimple and easy to use “direct index’, which although lacksthe integration of many of the variables, together withappropriate regulations that consider the effects of workintensity, acclimation, and clothing is advantageous overthe other indices. In this group, two indices are in daily use

Fig 1. The correlation between the WBGT and DI values (n=108).


for more than four decades: the WBGT index and the DI.The WBGT index was adopted by many internationalestablishments, but is cumbersome to use. The DI, althoughit does not account directly for radiation, is easy to use andis in use in Israel very satisfactorily. It is suggested to adoptthis index and test its applicability also by others.

References

1) Wing JF (1965) Upper thermal tolerance limits for unimpairedmental performance. Aerospace Med 36, 960–64.

2) Allnutt MF, Allan JR (1973) The effects of core temperatureelevation and thermal sensation on performance. Ergonomics16, 189–96.

3) Epstein Y, Keren G, Moisseiev J, Gasko O, Yachin S (1980)Psychomotor deterioration during exposure to heat. AviatSpace Environ Med 51, 607–10.

4) Adolph EF and associates (1969) Physiology of man in thedesert, Hafner Publ Comp, New York.

5) Shibolet S, Lancaster MC, Danon Y (1976) Heat stroke: areview. Aviat Space Environ Med 47, 280–301.

6) Bell PA (1981) Physiological, comfort, performance, andsocial effects of heat stress. J Soc Issues 37, 71–94.

7) Belding HS, Hatch TF (1955) Index for evaluating heat stressin terms of resulting physiological strain. Heat Pip Air Condit27, 129–36.

8) Shapiro Y, Epstein Y (1984) Environmental physiology andindoor climate—thermoregulation and thermal comfort.Energy Build 7, 29–34.

9) Fanger PO (1970) Thermal comfort, Danish Technical Press,Copenhgen.

10) Goldman RF (2001) Introduction to heat-related problemsin military operations. In: Textbook of military medicine:medical aspects of harsh environments Vol. 1, Pandolf KB,Burr RE (Eds.), 3–49, Office of the Surgeon General,Department of the Army, Washington DC.

11) ASHRAE (1966) Thermal comfort conditions, ASRAEstandard 55.66, New York.

12) ISO 7730 (1984) Moderate thermal environments—determination of the PMV and PPD indices and specificationof the conditions for thermal comfort. ISO, Geneva.

13) ASHRAE (1992) Thermal environmental conditions forhuman occupancy. ANSI/ASHRAE standards, Atlanta.

14) Hensel H (1981) Thermoreception and temperature regulation,176, Academic Press, London.

15) Goldman RF (1988) Standards for human exposure to heat.In: Environmental ergonomics, Mekjavic IB, Banister EW,Morrison JB (Eds.), 99–138, Taylor & Francis, Philadelphia.

16) Bedford T (1936) The warmth factor in comfort at work: aphysiological study of heating and ventilation. IndustrialHealth Research Board No 76, HMSO, London.

17) Rohles FJ, Levins R (1971) The nature of thermal comfortfor sedentary man. ASHRAE Trans 77, 239–46.

18) Parsons K (2003) Human thermal environments, 2nd Ed.,

258–92, Taylor & Francis, London.19) Haldane JS (1905) The influence of high air temperature J

Hyg 5, 494–513.20) Belding HS (1970) The search for a universal heat stress

index. In: Physiological and behavioral temperatureregulation, Hardy JD, Gagge AP, Stolwijk JAJ (Eds.), 193–202, Charles C Thomas, Springfield.

21) Gagge AP, Nishi Y (1976) Physical indices of the thermalenvironment. ASHRAE J 18, 47–51.

22) NIOSH (1986) Criteria for a recommended standard:occupational exposure to hot environment. DHHS (NIOSH)Publication No 86–113, 101–10, National Institute forOccupational Safety and Health, Washington DC.

23) Houghton FC, Yaglou CP (1923) Determining equal comfortlines. J Am Soc Heat Vent Engrs 29, 165–76.

24) Vernon HM, Warner CG (1932) The influence of the humidityof the air on capacity for work at high temperatures. J Hyg32, 431–62.

25) Yaglou CP, Minard D (1957) Control of heat causualties atmilitary training centers. Am Med Ass Arch Ind Hlth 16,302–16.

26) NIOSH (1972) Occupational exposure to hot environment.National Institute for Occupational Safety and Health, HSM72-10269. Department of Health, Education, and Welfare,Washington DC.

27) ISO 7243 (1982) Hot environments—estimation of the heatstress on working man, based on the WBGT-index (wet bulbglobe temperature). ISO, Geneva.

28) ACGIH (2004) TLVs and BELs. Threshold limit values forchemical substances and physical agents and biologicalexposure indices, 168–76, ACGIH Signature Publications,Cincinnati.

29) US Department of Labor (1999) OSHA Technical manual(OTM) (TED 01-00-015), section III chapter 4: heat stress,US Department of Labor, Washington DC.

30) AIHA (1975) Heat exchange and human tolerance limits.In: Heating and cooling for man in industry, 2nd Ed., 5–28,American Industrial Hygiene Association, Arkon.

31) Armstrong LE, Epstein Y, Greenleaf JE, Haymes EM,Hubbard RW, Roberts WO, Thompson PD (1996) ACSMPosition stand: heat and cold illnesses during distance running.Med Sci sports Exerc 28, i–x.

32) Department of the Army (1980) Prevention, treatment, andcontrol of heat injury, 1–21, Technical Bulletin No TB Med507, Department of the Army, Washington DC.

33) Moran DS, Pandolf KB (1999) Wet bulb globe temperature(WBGT)—to what extent is GT essential. Aviat Space EnvironMed 70, 480–4.

34) Lind AR, Hallon RF (1957) Assessment of physiologicseverity of hot climate. J Appl Physiol 11, 35–40.

35) Goldman RF (1973) Environmental limits, their prescriptionand proscription. Int J Environ Studies 5, 193–204.

36) Thom EC (1959) The discomfort index. Weatherwise12, 57–60.

37) Sohar E, Adar R, Kaly J (1963) Comparison of the



environmental heat load in various parts of Israel. Bull ResCounc Israel 10E, 111–5.

38) Sohar E, Tennenbaum DJ, Robinson N (1962) A comparisonof the cumulative discomfort index (Cum DI) and cumulativeeffective temperature (Cum ET), as obtained bymeteorological data. In: Biometeorology, Tromp SW (Ed.),395–400, Pergamon Press, Oxford.

39) Tennenbaum J, Sohar E, Adar R, Gilat T, Yaski D (1961)The physiological significance of the cumulative discomfortindex (Cum DI).Harefuah 60, 315–9.

40) Sohar E (1979) Man in the desert. In: Arid zone settlementplanning—the Israeli experience, Golani G (Ed.), 477-518,Pergamon Press, New York.

41) Shapiro Y, Seidman DS (1990) Field and clinicalobservations of exertional heat stroke patients. Med SciSports Exerc 22, 6–14.

42) Epstein Y, Moran DS (2004) Extremes of temperature andhydration. In: Travel Medicine, Keystone JS, Kozarsky PE,Freedman DO, Nothdurft HD, Conner BA (Eds.), 383–92,Mosbey, Edinburgh.

43) Sohar E (1982) Men, microclimate and society. Energy Build4, 149–54.

44) IUPS Thermal Commission (2001) Glossary of terms forthermal physiology. Jap J Physiol 51, 245–80.

45) Goldman RF (1982) Thermal comfort in an era of energyshortage. In: Selected sensory methods: problems andapproaches to measuring hedonics, Kuznicki JT, JohnsonRA (Eds.), 64–98, Am Soc Test Materials, Phyladelphia.

46) Gonzalez RR, Gagge AP (1973) Magnitude estimates ofthermal discomfort during transients of humidity and operativetemperature (ET*).ASHRAE Trans 79, 89–96.

47) Hill L, Griffith O, Flack M (1916) The measurement of therate of heat loss at body temperature by convection, radiationand evaporation. Phil Trans Royal Soc 207(B), 183–220.

48) Dufton AF (1929) The eupatheostat. J Sci Instrum 6, 249–51.49) Winslow CEA, Herrington LP, Gagge AP (1938)

Physiological reactions and sensations of pleasantness undervarying atmospheric conditions. Trans ASHVE 44, 179–96.

50) Ionides M, Plummer J, Siple PA (1945) The thermalacceptance ratio. Interm report No 1, Climatology andEnvironmental Protection section US OQMG.

51) Robinson S, Turrell ES, Gerking S D (1945) Physiologicallyequivalent conditions of air temperature and humidity. AmJ Physiol 143, 21–32.

52) Bedford T (1946) Environmental warmth and itsmeasurement. Med Res Council Memo 17. HMSO, London.

53) McArdle B, Dunham W, Holling HE, Ladel WSS, Scott JW,Thomson ML, Weiner JS (1947) The prediction of thephysiological effects of warm and hot environments. MedRes Council, London RNP Report 47/391, London.

54) Missenard A (1948) A thermique des ambiences: équivalencesde passage, équivalences de séjours. Chaleur Indust 276, 159–72 (in French).

55) Craig (1950) Relation between heat balance and physiologicalstrain in walking men clad in ventilated impermeable

envelope. Fed Proc 9, 26.56) Lee DHK (1958) Proprioclimates of man and domestic

animals. In: Climatology, Arid zone research—X, 102–25,UNESCO, Paris.

57) Hall JFK, Polte W (1960) Physiological index of strainand body heat storage in hyperthermia. J Appl Physiol15, 1027–30.

58) Givoni B (1962) The influence of work and environmentalconditions on the physiological responses and thermalequilibrium of man. In: Proceedings of UNESCO Symposiumon Environmental Physiology and Psychology in AridConditions, 199–204, Lucknow.

59) McKarns JS, Brief RS (1966) Nomographs give refinedestimate of heat stress index. Heat Pip Air Cond 38, 113–6.

60) Fuller FH, Brouha L (1966) New engineering methods forevaluating the job environment. ASHRAE J 8, 39–52.

61) Gagge AP, Rapp GM, Hardy JD (1967) Effective radiant fieldand operative temperature necessary for comfort with radiantheating. ASHRAE Trans 73, 2.1–9.

62) Lind AR (1970) Effect of individual variation on upper limitof prespective zone of climates. J Appl Physiol 28, 57–62.

63) Gagge A, Stolwijk A, Nishi Y (1971) An effective temperaturescale based on a simple model of human physiologicalregulatory response. ASHRAE Trans 77, 247–57.

64) Botsford JH (1971) A wet globe thermometer forenvironmental heat measurement. Am Ind Hyg Assoc J32, 1–10.

65) Nishi Y, Gagge AP (1971) Humid operative temperature. Abiophysical index of thermal sensation and discomfort. JPhysiol (Paris) 63, 365–8.

66) Givoni B, Goldman RF (1972) Predicting rectal temperatureresponse to work, environment, and clothing. J Appl Physiol32, 812–22.

67) Kerslake DM (1972) The stress of hot environment.Cambridge University Press, Cmbridge.

68) Gonzalez RR, Nishi Y, Gagge AP (1974) Experimentalevalution of standard effective temperature: a newbiometeorological index of man’s thermal discomfort. Int JBiomrtrorol 18: 1–15.

69) Givoni B, Pandolf RR (1973) Predicting heart rate responseto work, environment and clothing. J Appl Physiol 34, 201–4.

70) Gonzalez RR, Bergulnd LG, Gagge AP (1978) Indices ofthermoregulatory strain for moderate exercise in the heat. JAppl Physiol 44, 889–99.

71) Nunneley SH, Stribley F (1979) Fighter index of thermalstress (FITS): guidance for hot-weather aircraft operations.Aviat Space Environ Med 50, 639–42.

72) Kamon E, Ryan C (1981) Effective heat strain index usingpocket computer. Am Ind Hyg Assoc J 42, 611–5.

73) Shapiro Y, Pandolf KB, Goldman RF (1982) Predicting sweatloss response to exercise, environment and clothing. Eur JAppl Physiol Occup Physiol 48, 83–96.

74) ISO 7933 (1989) Hot environments—analytical determinationand interpretation of thermal stress using calculation of


required sweat rate. ISO, Geneva.75) Gagge AP, Fobelets AP, Berglund LG (1986) A standard

predictive index of human response to the thermalenvironment. ASHRAE Trans 92, 709–31.

76) Frank A, Moran D, Epstein Y, Belokopytov M, Shapiro Y(1996) The estimation of heat tolerance by a new cumulativeheat strain index. In: Environmental Ergonomics: Recentprogress and new frontiers, Shapiro Y, Moran D, Epstein Y(Eds.), 194–7, Freund Pub House, London.

77) Moran DS, Shitzer A, Pandolf KB (1998) A physiological strainindex to evaluate heat stress. Am J Physiol 275, R129–34.

78) Moran DS, Shapiro Y, Epstein Y, Matthew W, Pandolf KB(1998) A modified discomfort index (MDI) as an alternativeto the wet bulb globe temperature (WBGT). In: EnvironmentalErgonomics VIII, Hodgdon JA, Heaney JH, Buono MJ (Eds.),

77–80, Int Conf Environ Ergo, San Diego.79) Moran DS, Pandolf KB, Shapiro Y, Heled Y, Shani Y, Matthew

WT, Gonzales RR (2001) An environmental stress index (ESI)as a substitute for the wet bulb globe temperature (WBGT).J Thermal Biol 26, 427–31.

80) Wallace RF, Kriebel D, Punnett L, Wegman DH, WengerCB, Gardner JW, Gonzales RR (2005) The effects ofcontinuous hot weather training on risk of exertional heatillness. Med Sci Sports Exerc 37, 84–90.

81) USARIEM (2005) Heat injury prevention program. Appendix1: Commander’s, senior NCO’s, and instructor’s guide torisk management of heat casualties. Available from: http://www.usariem.army.mil/HealthInjury.htm. Accessed April,2005.

Appendix

Current guidelines of working/exercising under the various levels of heat load as specified in terms of

WBGT index (°C)

American College of Sports Medicine (ACSM)31)

Level of risk WBGT (°C)

Very high Above 28

High 23–28

Moderate 18–23

Low Below 18

The risk of heat illness for runners while wearing

shorts, socks, shoes and t-shirt.

American Conference of Governmental Industrial Hygienists (ACGIH)28)

acclimated non-acclimated

Work demands L M H VH L M H VH

100% work 29.5 27.5 26.0 27.5 25.0 22.5

75% work; 25% rest 30.5 28.5 27.5 29.0 26.5 24.5

50% work; 50% rest 31.5 29.5 28.5 27.5 30.0 28.0 26.5 25.0

25% work; 75% rest 32.5 31.0 30.0 29.5 31.0 29.0 28.0 26.5

Work demands: L=light work; M=moderate work; H= heavy work; VH=very heavy work.

WBGT additions for clothing type are as follows: summer work uniform=0; woven material

overalls=+3.5; double- cloth overall= +5.



Guidelines in Israel as specified by DI40)

Level DI Significance

Light 22–24 Mild sensation of heat

Moderate 24–28 Physical work is performed with some difficulties

Severe >28 Body temperature cannot be maintained during physical work.

High risk for heat illness

For workers dressed in light summer clothing.

15 min rest during each hour of exercise. Under severe heat load physical work is not tolerable.

Fluid consumption ml/h in regard to heat stress and work intensity42)

US Department of the Army81)

Easy work Moderate work Hard work

Heat WBGT Work/rest Water Work/rest Water Work/rest Watercategory intake intake intake

(flag) (°C) (min) (ml/h) (min) (ml/h) (min) (ml/h)

1 25.6– NL 500 NL 750 40/20 750(White) 27.7 (70)*

2 27.8– NL 500 50/10 750 30/30 1000(Green) 29.4 (150) (65)

3 29.5– NL 750 45/15 750 30/30 1000(Yellow) 31.0 (100) (55)

4 31.1– NL 750 30/30 750 20/40 1000(Red) 32.1 (80) (50)

5 >32.2 50/10 1000 20/40 1000 10/50 1000(Black) (70) (45)

The table refers to heat acclimated soldiers wearing battledress uniform (BDU).

The work-rest times and fluid replacement volumes will sustain performance and hydration for at

least 4 h of work in the specified heat category.

Fluid needs can vary based on individual differences (± 250 ml/h) and exposure to full sun or full

shade (± 250 ml/h).

NL=no limit to work time per hour.*=continuous work (add 250 ml/h to fluid consumption).

If wearing body armor add 2.5°C to WBGT in humid climates. If wearing NBC clothing (mission-

oriented protective posture (MOPP 4)), add 5°C to WBGT index for easy work, and 10°C to WBGT

index for moderate and hard work.

Work intensityHeat load (DI units)

Light Moderate Severe

Rest 50 100 200

Light 400 500 600

Moderate 500 700 800

Heavy 850 1,000* 1,250*

Add 300 ml/h for working in the sun.*Theoretical values, since physical work is not tolerable.

Thermal Comfort and the Heat Stress Indices

Documents

Transcript of Thermal Comfort and the Heat Stress Indices